Vibration sensors have been used in a variety of areas such as oil and gas exploration, vibration monitoring of buildings, bridges and other civil constructions. As vibration sensors for seismic exploration, earthquake and building vibration monitoring are usually powered by batteries, vibration sensors with low power consumption are generally preferred. Also, it is preferable that vibration sensors are low cost and reliable, and have a wide frequency bandwidth.
Conventional geophones are a type of vibration sensors having been widely used for many years. Conventional geophones have a coil movable in a magnetic field. Movement of the coil triggered by external vibration develops an electronic voltage across the coil terminals, which may be used for determining the characteristics of the external vibration.
For example, European Patent Publication No. 0,110,431 teaches an acceleration-responsive geophone of the type employing a transducer including a sensor coil and a drive coil which are both disposed in a magnetic field produced by a magnet structure. The magnet structure and the coils are mounted within a housing for movement relative to each other. The magnet structure is arranged to reduce the electromagnetic coupling between the sensor coil and the drive coil to substantially zero. The sensor coil is coupled to the input of an electronic amplifier having its output coupled to the drive coil to provide a feedback circuit. The transducer-amplifier combination has the behaviour of a bandpass filter. In order to render the combination substantially temperature-independent, while maintaining its bandpass characteristics over a wide temperature range, the amplifier is a transconductance amplifier having an input impedance and an output impedance which are highly relative to the impedance of the sensor coil and the impedance of the drive coil, respectively. A substantially temperature-independent resistor is connected in series to the drive coil, and connected to an output terminal via which the output signal of the transducer-amplifier can be collected.
U.S. Pat. No. 5,172,345, also published as European Patent No. 0,434,702 and PCT Patent Application No. PCT/NL89/00063, teaches a geophone system for measuring mechanical vibrations such as seismic waves. The geophone system includes a mechanical transducer with an electronic processing circuit. The mechanical transducer includes an inertial mass adapted to be excited by an input acceleration signal and by a force transducer. The excitation is detected by a sensor element and the processing circuits of the geophones control the force transducer and are connected with a central station via a transmission line.
Conventional geophones are low cost, power efficient and reliable. However, their frequency bandwidth is generally narrow (frequency response dropping approximately 12 dB/octave), and their total harmonic distortion (THD) is generally high (approximately 0.1% or −60 dB), rendering them unsatisfactory in the evolving market.
Conventional geophones usually have poor frequency response at low frequency range. As low frequency seismic signals are becoming more commonly used in the seismic industry, for instance, vibrator sweeping frequency now usually starting at approximately 2 Hz or lower, the conventional geophones do not meet the needs of monitoring low frequency vibrations. A sensor with wide bandwidth, in particular with good frequency response in low frequency range, is therefore desired.
Other vibration sensors, such as open-loop and closed-loop micro electromechanical systems (MEMS) sensors, are also available. Based on the sensor structure, they are categorized into two classes: open-loop vibration sensors and closed-loop vibration sensors. The conventional geophones are also open-loop vibration sensors.
Similar to conventional geophones, open-loop vibration sensors are generally economic, reliable, and power efficient. Some open-loop vibration sensor arrangements do not even need a power supply at all, although open-loop MEMS sensors do require power and are an example of an exception to the generalization of being power efficient. However, open-loop vibration sensors generally have a very limited frequency bandwidth and poor THD dampening qualities.
Comparing to open-loop vibration sensors, the closed-loop vibration sensors, such as closed-loop MEMS sensors, have generally larger bandwidth with a range of approximately 3 to 375 Hz and lower THD of approximately 0.001% or −100 dB. However, these sensors are expensive and fragile, rendering them unreliable in some use scenarios.
Moreover, closed-loop vibration sensors are power inefficient. For example, the power consumption of a closed-loop MEMS sensor may be as high as 125 mw or higher. The relatively high power consumption requirement severely prevents closed-loop MEMS sensors from successful entry into the seismic market.
Therefore, there is a desire for a vibration sensor that has small total harmonic distortion, a wide frequency bandwidth with good frequency response at low frequencies, and low power consumption.